1. Field of the Invention
This invention relates to the field of nuclear scintillation cameras, and more particularly to systems and methods for improving uniformity of imaging response of such cameras.
Radiation scintillation cameras are useful in medical diagnostics. Such cameras detect the presence and distribution of radio-active tracers which are injected into patients and which have characteristic affinity, depending on the kind of tracer used, for certain organs and tissue types. By detecting the tracer and producing an image of the radiation pattern generated by the tracer within the selected organ or tissue, an image describing characteristics or condition of the organ or tissue is produced.
Commonly used scintillation cameras include a unit for detecting radiation events from within the patient, stimulated by the radio-active tracer, and downstream imaging electronics and display apparatus coupled to the detector for producing a visual image of the pattern of the radio-active tracer within the patient's body.
The detector includes a scintillation crystal, which is responsive to radiation events impinging upon it to produce a light flash, or scintillation, within the crystal in the vicinity of where the radiation strikes it. An array of photomultiplier tubes is disposed to view the crystal. The photomultiplier tubes convert the light flash of each scintillation event to an electrical signals. A decoding matrix is connected to the phototube array and converts each set of electrical signals from the photomultiplier tubes to a radiation count signal indicating both the location (on an x--y plane) and energy level (brightness) of the scintillation event in response to which the radiation count signal is produced.
No scintillation camera is capable of producing radiation count signals which define either the location or the energy level of radiation events with absolute precision. Minute inaccuracies in the location, and energy level decoding cause nonuniformity in the image count density of the scintillation camera systems.
The uniformity of image count density of a scintillation camera detector can be degraded by factors including inaccuracies in energy level decoding, inaccuracies in location (x-y) decoding, (nonlinearity), regional variations in detector sensitivity, and other factors.
Incorrect energy level decoding is caused in part by the fact that the sensitivity of photomultiplier tubes to a scintillation depends upon the angular orientation of the scintillation relative to the tube axis. Accordingly, even if the photomultiplier tubes are perfectly tuned, it is possible to perfectly adjust them for proper decoding only at a finite number of discreet field locations corresponding to the number of photomultiplier tubes used in the camera system. A large proportion of the detector field is thus non-adjustable, and subject to the degree of non-uniformity of response of each individual photomultiplier tube and the response of each region of the scintillation crystal (which may also have nonuniform response to radiation as a function of location).
Tubes and crystals can thus be highly non-uniform in their response to even a uniform flux of gamma rays input to the crystal.
Incorrect energy level decoding can be exaggerated by electrical drift in the gain of individual photomultiplier tubes which can occur with time.
In addition to adversely affecting energy level decoding uniformity in a general fashion, errors in the energy decoding also render extremely critical the adjustment of pulse height analyzer windows used in the camera, since any degree of error in the adjustment of a window accentuates the incorrect energy level decoding.
Incorrect location decoding (non-linearity) also contributes to deterioration of imaging uniformity. Incorrect location decoding is caused by all of the factors which can contribute to incorrect energy level decoding. When this happens, more or less radiation counts are placed per unit area in different regions of the field, even in response to uniform radiation. The image count density is thus made nonuniform. These regional linearity perturbations need only be tiny to cause large changes in imaging uniformity, because their effect upon image count density is squared. This fact stems from the fact that the linearity perturbations are usually superimposed, because the same tubes and crystal are used to decode both the "x" and "y" location co-ordinates.
Various other factors adversely affect uniformity. A partial enumeration of these are collimator regional sensitivity changes, regional variations in crystal light conversion efficiency changes, and gamma detection efficiency, and electronically caused non-linearity and regional sensitivity variations.
2. Description of the Prior Art
Proposals have been made to compensate or correct for non-uniformity in scintillation camera imaging. One such proposal involves the digitization and recording of information produced by a scintillation camera and describing radiation images. The information for an entire study is digitized and recorded on magnetic tape. After completion of the study, the stored information is input to a digital computer which analyzes and manipulates the information in accordance with certain algorithms. The algorithms are chosen such that the processed information is corrected for imaging uniformity. The processed information is then reconverted to analogue form by display apparatus, and images corresponding to radiation detected in the earlier study can be observed.
Operation in accordance with this proposal is slow, and requires highly expensive, complex and bulky equipment, including a digital computer.
The results of uniformity corrected studies cannot be viewed directly. Rather, such results are available only after recording, processing and reconversion of the imaging information to analogue form. The requirement for complex digital computing equipment and its programming is highly expensive and requires skilled setup and operation personnel.
Another proposal for uniformity correction has been made which does not require the use of complex digital computational equipment. In this more recent proposal, only regional image count density errors are corrected. In such a proposal, the camera is preconditioned by exposure to a uniform gamma radiation flux over its field of view prior to the initiation of a study. The response of the camera to this uniform radiation is measured over several different regions of the field, and the measured response for each region is stored. This stored sensitivity information is used to cause the camera to discard some of the counts from high sensitivity regions, to equalize the response of the camera in its high sensitivity regions with the response in regions of lower sensitivity.
This proposal for uniformity correction has at least two disadvantages. One disadvantage is that it corrects, as mentioned above, only for imprecisions in regional image sensitivity, and fails to correct for energy level inaccuracies. A second disadvantage, perhaps more important, is that, by discarding a substantial number of radiation counts, this uniformity correction proposal reduces camera sensitivity considerably. The reduction in sensitivity lengthens the time required for performing diagnostic studies with the camera, and impairs resolution of radiation images produced by the camera, which obviously impairs the camera's effectiveness.